1. Field of the Invention
The present invention relates to an apparatus for transferring an original pattern drawn on a mask onto a surface to which transfer is to be made (hereinafter referred to as "transfer surface") by using a charged particle beam such as an electron beam, an ion beam, etc.
2. Description of the Related Art
Recently, the semiconductor integrated circuit technology has made remarkable progress, and there has been a marked tendency for semiconductor devices to become smaller in size and higher in integration degree. As a lithography system for printing an integrated circuit pattern on a semiconductor wafer, a so-called optical stepper, which uses light, has generally been employed. However, as integrated circuit patterns become finer, the demanded resolution becomes higher, and thus the resolution limit of optical lithography has become a matter of concern. Under these circumstances, lithography systems that use an electron beam, an ion beam, X-rays, etc. have been actively investigated and developed in recent years. Among these radiations, particularly the electron radiation is considered to be most practical, and various electron beam lithography systems have been proposed and developed. Hitherto, electron beam lithography systems have been used in pattern writing for masks and reticles by virtue of the excellent controllability thereof. However, the conventional electron beam exposure systems are designed to write patterns one by one on a radiation-sensitive substrate unlike optical steppers in which pattern transfer is effected by using a mask. Therefore, the conventional electron beam exposure systems involve a long exposure time and have not heretofore been used for lithography of production model wafers from the viewpoint of device production cost.
Electron beam exposure systems that use a mask have also been under development and investigation, and there has been devised a perforated stencil mask having a structure as shown in FIG. 5, in which a base plate BP which blocks an electron radiation is provided with openings OP. The stencil mask suffers, however, from the problems: 1 it is impossible to form a doughnut-shaped pattern because no annular opening OP can be provided; and 2 a large amount of heat is generated from the mask because the applied electron beam is completely absorbed by the base plate BP. Therefore, the stencil mask could not be put to practical use. However, a scattering mask shown in FIGS. 6(a) and 6(b) has recently be proposed as a mask that solves the above-described demerits of the stencil mask. As shown in FIGS. 6(a) and 6(b), the scattering mask 1 has a thin support film 2 which transmits almost all the applied electron beam or scatters forwardly the electron beam at a small angle, and a scattering film 3 is disposed on the upper side of the support film 2. As shown in FIG. 6(b), an electron beam is batch-wise applied to a proper region on the scattering mask 1. Consequently, an electron beam EB2 (shown by the chain double-dashed line in the figure) passing through the scattering film 3 is scattered forwardly to a larger extent than an electron beam EB1 (shown by the solid line in the figure) passing through only the support film 2. An electron beam reduction transfer apparatus is provided with a pair of projection lenses 5 and 6 for projecting a pattern drawn on the scattering mask 1 onto a wafer W as a demagnified image, and a scattering aperture plate 7 disposed between the projection lenses 5 and 6. The scattering aperture plate 7 is provided with an aperture 7a for passing only the electron beam in the vicinity of the center of a crossover image CO formed by the projection lens 5. Because of the difference in the degree of scattering of the electron beam caused by the support film 2 and the scattering film 3, the electron beam EB2 passing through the scattering film 3 is higher than the electron beam EB1 passing through only the support film 2 in terms of the ratio in which the electron beam is intercepted by the scattering aperture plate 7. Thus, the resist coated on the wafer W is exposed in a pattern corresponding to the gap (whited-out portion in FIG. 6(a)) between a pair of adjacent patterns of the scattering film 3. It should be noted that in a system wherein the central portion of the scattering aperture plate 7 is formed as a blocking portion, and an opening is provided around the central portion, the resist is exposed in a pattern corresponding to the pattern of the scattering film 3 in reverse relation to the above.
The electron beam exposure method using the above-described scattering mask is expected to be satisfactorily used for the lithography of semiconductor integrated circuits in the future in terms of both resolution and throughput. In the scattering mask system, since the scattering film 3 can be supported by the support film 2, (i) it is possible to form a doughnut-shaped pattern in which an insular scattering film 3 is present as shown in part A of FIG. 6(a). In addition, (ii) since the applied electron beam need not be completely blocked by the scattering film 3, and hence the amount of electron radiation blocked by the mask portion is reduced, it is possible to suppress the generation of heat in the mask portion by the electron beam irradiation. That is, the above-described problems 1 and 2 associated with the perforated stencil mask (shown in FIG. 5) are solved.
However, there is a limit to the reduction of the thicknesses of the support and scattering films 2 and 3 of the scattering mask 1, shown in FIGS. 6(a) and 6(b), from the viewpoint of the practical thin film forming technology. That is, in view of the uniformity of film thickness and the film strength, the thickness of each of the support and scattering films 2 and 3 cannot be reduced beyond several tens of nanometers. In order to obtain the above-described effect of the scattering mask at such a film thickness, an electron beam must pass through the mask at a speed of at least about 100 keV. However, if the speed of the electron beam is raised to such a high level, other demerits arise from the viewpoint of exposure: (1) the resist sensitivity reduces; and (2) since the energy applied to a surface to be exposed becomes large, the resist and the wafer generate heat, thereby causing the properties thereof to change.